Batteries are the most common electrical energy storage devices for electric vehicle, medical instruments, uninterruptible power supply, and other industrial applications. In use, when a potential higher than a basic potential of a battery is needed, it is common to use a plurality of batteries which are connected in series. However, even though the batteries are produced with the same structure via a typical production method using the same anode, cathode and electrolyte material, a difference in charging or discharging (and self-discharging) characteristics exists between each of the batteries connected in series. Thus there is a potential difference between the series-connected batteries, which may cause a problem of unbalanced charging. For example, as the potentials of the series-connected batteries are different from each other, some batteries may reach a prescribed voltage when some other batteries do not reach the prescribed voltage, and therefore an overcharge or undercharge problem or even both overcharge and undercharge problems may be caused during charging the series-connected battery string.
An overcharged battery may have high risk of explosion if the chemistry of the battery cannot withstand overcharged state, whereas an undercharged battery may reduce the life cycle of the series-connected battery string. Therefore, it is needed to address the unbalanced charging problem for the series connected battery string. A common method is to use charge equalization devices to achieve charge equalization for the series-connected battery string.
A conventional passive approach 10 for achieving charge equalization is represented in FIG. 1. In the illustrated series-connected battery string 12, each battery 14 is connected with a parallel resistor 16, which assists to achieve charge equalization for each battery 14. It is a simple low cost way, but it suffers high energy loss and heat dissipation. Moreover, the charge equalizer capability is limited for batteries of high current.
A conventional active approach 20 for achieving charge equalization is represented in FIG. 2. In the illustrated series-connected battery string 22, each battery 24 is provided with a power electronics circuit 26 for achieving charge equalization. The power electronics circuit 26 may be a combination of switches, diodes, inductors, and/or capacitors. It is a method of high energy efficiency, and it is able to achieve charge equalization for batteries of high current. However, as the individual power electronics circuit for achieving charge equalization is high in cost and large in size, there are problems in both implementation cost and size.